The present invention is directed to the field of demodulators and more particularly to a spread spectrum demodulator for demodulating a spread spectrum four-phase PSK (phase shift keyed) modulated carrier signal.
In a bi-phase PSK communication system the phase of a reference carrier signal is shifted (encoded) in accordance with the coding of a data signal. As an example, if a binary "0" is to be transmitted the phase of the reference carrier signal would be unshifted, or 0.degree.; and if a binary "1" were to be transmitted the phase of the reference carrier signal would be shifted 180.degree..
Once the carrier signal is phase modulated it may be transmitted to a receiver over a transmission line or a radio link.
Aside from bi-phase PSK modulation, quadrature (four) phase modulation may also be used. In a quadrature phase communication system the information signal, which is generally a string of binary bits, is proportioned into baud intervals, that is, into groups of two binary bits. The succession of phase changes occurring in successive baud intervals is then used to modulate the reference carrier signal in four phases.
For example, four digital symbols such as 00, 01 10 and 11 may be transmitted for quadrature phase modulation of the carrier signal. Each of the four different phases of the carrier signal may be used to represent a different one of the four digital symbols. That is, the phase angle 0.degree. can represent the digital symbol, 00; and the phase angles 90.degree., 180.degree., and 270.degree., can represent the digital symbols 01, 11 and 10, respectively.
In certain communication environments, it is necessary to provide a signal which cannot be interfered with, that is, a signal which is secure and cannot be jammed, interrupted, or received by an unauthorized receiver.
The security afforded to a PSK signal is somewhat limited in that a receiver, having its local reference carrier signal synchronized to the received PSK signal, will be able to demodulate the received PSK signal to arrive at the data signal. In addition, once the frequency of the carrier signal or the frequency spectrum of the data signal is known, it is a relatively simple procedure for a jammer to intentionally disrupt communications. In order to minimize this particular weakness in digital communications systems a scheme has been devised whereby the bandwidth of the transmitted signal is spread over a larger bandwidth than that of the data signal. This is generally accomplished by mixing the data signal with a pseudo-random sequence of pulses having a bandwidth greater than the bandwidth of the data. The mixed signal is then used to phase modulate a reference carrier signal. Transmission of this type of spread spectrum signal makes it exceedingly difficult to determine the data carrying components of the transmitted signal and therefore, in turn, to achieve effective jamming. In order to receive a spread spectrum signal that is encoded in the above manner, it is necessary to know the particular sequence of the pseudo-random sequence of pulses that was used to spread the spectrum of the data signal. The present invention is directed towards such a system. Efficient decoding in the receiver requires an identical code generator for generating the pseudo-random sequence, which code generator must be in precise time synchronization with the pseudo-random sequence of the spread spectrum signal.
A publication which provides an insight into the state of the art of spread spectrum signal processing is entitled "Surface Acoustic Wave Devices And Applications," by B. J. Hunsinger, published in the November 1973 issue of Ultrasonics, page 25 to page 263. Another publication of interest is entitled "Optimal Binary Sequences For Spread Spectrum Multiplexing, " by R. Gold published in the IEEE Transactions On Information Theory, October 1967, pages 619 to 621.